Antimicrobial therapy has not effectively minimized the significant morbidity and mortality associated with infections due to extra-intestinal isolates of Escherichia coli. Optimal therapy hinges on understanding their pathogenesis. Capsular antigens of E. coli have been variably suggested as being important virulence factors, with established serotypes being broadly categorized into groups 1 or 2. Data from our laboratory has established the group 2 capsule K54 as being an important virulence factor. Therefore, we began an evaluation of the regulation of the group 2, K54 capsular polysaccharide. As a starting point, we assessed the role of the group 1 capsule regulators RcsA,RcsB and Lon on the regulation of group 2 capsule production. We have established the CP9, an extra-intestinal isolate of Escherichia coli can be induced to produce a group 1 capsular polysaccharide in addition to its constitutive production of a group 2, K54 capsule. The group 1 positive regulators, RcsA and RcsB, and the negative regulator Lon protease are present and act on group 1 capsule production as previously described for K-12 and K3O strains. The effect of these regulators on the group 2, K54 capsule was assessed by two separate methods that utilized various CP9 constructs with disruptions of rcsA, rcsB, Ion, rcsA and Ion and rcsB and Ion. The first method measured changes in activity of two independent group 2, K54 capsule gene protein fusions. The second method evaluated the serum sensitivity of these strains. These results established that RcsA serves as a negative regulator of group 2 capsule production in a functionally significant manner. RcsA's negative regulatory activity on the group 2, K54 capsule is greatest at 28 degrees C and is RcsB dependent. This negative regulatory effect decreases with a rise in temperature. The existence of another Lon protease sensitive, negative regulator of the K54 capsular polysaccharide was also suggested. This project has been terminated at N.I.H.